Sabo defeating Akainu.
Makes no sense narratively and would completely ruin the entire build up to give the Ace copy cat the victory over Luffy, who has not only witnessed his brother being killed by Akainu infront of him and who suffered the greatest wound from Akainu afterwards, but Luffy felt responsible for what happend to Ace.
Akainu is literally the only character in the entire manga Luffy holds a personal grudge against- something he doesn't even feel torwards Teach: